In a fuel vapor treatment system, fuel vapor generated in a fuel tank is temporarily adsorbed by a canister. During an engine is operated, the fuel vapor is desorbed from the canister and purged into an intake pipe through a purge passage. The purged fuel vapor is combusted in a combustion chamber of the engine. Such a purge process regenerates an adsorbing capacity of the canister.
While the purge process is conducted, fuel injected by a fuel injector and the fuel vapor are introduced into the combustion chamber to be combusted. The fuel injection quantity is adjusted in consideration of the fuel vapor quantity in order to restrict a disturbance of an air-fuel ratio.
It is important to accurately detect the fuel vapor concentration at a vicinity of the fuel injector in order to restrict the disturbance of the air-fuel ratio. In a system shown in JP-2005-351216A (U.S. Pat. No. 7,007,684B2), a fuel vapor concentration at a vicinity of the fuel injector is estimated based on a transit time from when the purge valve is opened to when the fuel vapor reaches the fuel injector and a change in concentration of the fuel vapor at the vicinity of the fuel injector. More specifically, the fuel vapor concentration at the vicinity of the fuel injector is estimated based on an assumption that the fuel vapor concentration at the vicinity of the injector changes linearly with respect to elapsed time.
However, according to the research of the inventors, the fuel vapor concentration at the vicinity of the fuel injector does not linearly change with respect to the elapsed time. Hence, in the system shown in the above patent document, the fuel vapor concentration at the vicinity of the fuel injector cannot be estimated accurately. The disturbance of air fuel ratio cannot be reliably restricted.